dextral displacements along the Talas-Fergana. Fault (Burtman, 1973). .... sliver entraps a part of the Uliastai forearc region, the Lugingol-Hegenshan unit, which ...
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INSTlTUT DES SClEllCES DE LA TERRE D'ORltAHs
International Field Excursion and Workshop on Tectonic Evolution and Crustal Structure of the Paleozoic Chinese Tianshan Urumqi, China, September 9-19, 2009
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Organized by State Key Laboratory of Lithospheric Evolution Institute of Geology and Geophysics, CAS Xinjiang 305 Project
International Field Excursion and Workshop on Tectonic Evolution and Crustal Structure of the Paleozoic Chinese Tianshan Urumqi, China, September 9-19,2009
Permian-Triassic transcontinental shear zones in Southern Mongolia Boris A. Natal'in 1, Tomurhuu Dondov 2, Ariunchimeg Yarimpil 2 , and Otgonbaatar Dorjsuren 2 1 - Istanbul Technical University, Istanbul, Turkey, natalin(mitu.edu.tr ; 2 - Institute of Geology and Mineral Resources, Ulaanbaatar, Mongolia, dt(a1igmr.mas.ac.mn
During
the
Permian
(270-250
Ma),
the
2000 km
(~eng6r
and Natal'in, 1996). In Altay, a
evolution of the Altaids was controlled by the
foliation produced by this shearing yields 271 Ma
sinistral Irtysh-Gomostaev and dextral Scytho-
Ar-Ar age. In the Triassic, this shear zone was
Turanian shear zones (Fig.
I). The Irtysh-
reactivated with dextral sense of motions as it is
Gomostaev shear zone can be traced as the NW-
evidenced from en-echelon pattem of rifts and
trending narrow zone stretching from Chinese
grabens in the basement of the West Siberian
Altay to the Polar Urals where it cuts the
Basin (Natalin and 2006).
structures of the Ural ides. The displacement along the Irtysh-Gomostaev shear zone was estimated as
~eng6r,
2000; Allen et al.
Early-Permian - Triassic 250-210 Ma
[===~=l
Baykalides
~uralides
k>,~~j
Altaids
~ Manchurides
cratons
continental blocks extensional basins The Permo-Triassic Silk Road arc
Fig. 1 Transcontinental shear zones in Northem Asia during the Permian-Triassic (modified after Natal'in and
~eng6r,
2005).
The E-W trending Scytho-Turanian shear zone
of the Russian craton. In the west, its structure and
stretches from Beishan to the southeastem margin
kinematic history are poorly known because of a 44
International Field Excursion and Workshop on Tectonic Evolution and Crustal Structure of the Paleozoic Chinese Tianshan Urumqi, China, September 9-19, 2009
wide extent of younger sediments. In Western
displacement distr ibuted across a rather wide zone
Tien Shan, the motion along this zone produced
in the northern wall of the Scytho-Turanian Fault.
the Tul melange exposed in the northern slope of
In Mongolia, the Irtysh-Gornostaev and Scytho-
the Alay Range. Contrary to subduction-related
Turanian shear zones join each other making a
melanges the Tul melange contains Devonian to
complicated pattern of branching E-W-trending
Lower
of vanous
faults. Kinematic features of those lying on the
tectonic setting incorporated in a Late Silurian
continuation of the Irtysh-Gornostaev shear zone
matrix.
criteria of steep dipping
were inferred as sinistral from the kinematic
foliation of this matrix indicate dextral sense of
history of the Altaids and Manchurides (Sengor
shear. Farther to the east, near Vuadil Village, this
and Natal' in, 1996) however direct structural
fault zone forms a wide (2-4 km) belt of high-
observations supporting this inference appeared
strain rocks with steep dipping foliation and
later and only in a few locations (Badarch et ai,
horizontal stretching lineation. Late Carboniferous
2002). Our recent studies 30 km south of Hurmen
conglomerates deposited after the collision of the
(H in Fig.2), indicate that Upper Permian clastic
Alay microcontinent are stretched parallel with
rocks form a system of large scale (3 km)
that lineation. This imposes a lower time limit for
asymmetric folds with steep plunging hinges
the ductile dextral shear. The Permo-Triassic age
indicating dextral shearing along NWW-SEE
of faulting can be inferred from synchronous
strikes. This shearing was observed at the northern
dextral displacements along the Talas-Fergana
boundary of the Dzolen tectonic unit representing
Fault (Burtman, 1973). Similar to dextral Junggar
north-facing accretionary wedge paired with the
Fault it can be considered as Reidel shear to the
Paleozoic South Gobi arc (Fig. 2). Post-Permian
Scytho-Turanian Fault.
timing of this dextral shearing is synchronous with
Carboniferous Kinematic
inclusions
In the Chinese Tien Shan, the Central Tien Shan
the above mentioned reactivation of the Irtysh-
Fault is a segment of the Scytho-Turanian Fault.
Gornostaev shear zones and suggests its extent
There, the dextral ductile shearing yields 290 to
across Mongolia.
245 Ma Ar-Ar ages (Laurent-Charvet et aI., 2003).
Tectonic units of Southeastern Mongolia usualy
Recent paleomagnetic studies (Van der Voo et
defined as terranes despite the fact that many of
al.
2006)
have
40-90°
them reveal a common stratigraphic and magmatic
counterclockwise rotation of Permian declinations
record (Fig. 2). Among them there are fragments
in the southern limb of the Kazakhstan orocline.
of Neoproterozoic structures intruded by granites
The similarity of declination orientations in the
of 950 and 916 Ma and covered by Vendian-
northern limb of the orocline with the Baltica
Lower Cambrian carbonates and quartzites. These
reference frame and minor differences of Tarim
fragments are intruded by Silurian to Permian
declinations comparing with Baltica, preclude an
granites preserving roof pendants of arc volcanic
interpretation the orocline formation due to NS
rocks of the same age. Sengor and Natal'in (1996).
shortening (e.g. Alexeiev. 2008). Shearing along
interpret them as a back-stop and single magmatic
the Scytho-Turanian Fault is more plausible
arc
explanation.
of paleomagnetic
Neoproterozoic basement of this unit is also
declinations requires about 1000 km of dextral
known as Tsagaan Uul and Hatag Uul terranes
The
established
rotation
45
defined
as
the
South
Gobi
unit.
The
International Field Excursion and Workshop on Tectonic Evolution and Crustal Structure of the Paleozoic Chinese Tianshan Urumqi, China, September 9-19, 2009
•
EB
o .
O._.:::30-=60_ _1:roi::=:=::i180 __ 2~""'t.'"
South Gobi Unit - PCm back-stop and Paleozoic arc Hegeshan·Lugingol • south·facing accretionary wedge of the South Gobi Unit Solonker Zone - in the north. it is southfacing accretionary wedge of the South Gobi Unit
~ Dzolen - north-facing accretionary "" wedge of the South Gobi Unit
~ Gobi-Allay
and Gurvansaihan -
~ south-facing arc paired with
Tuva-Mongol ~ Hangai-Hantei Paleozoic ~ accretionary wedge
!]TIl Manchurides - Paleozoic north-
I.LW factng arc of the North China block -
major faults (dominantly strike-slip faulls) in Sou!hem Mongolia
Fig. 2. Tectonic structure of Southern and Inner Mongolia. The Permian-Early Triassic Solonker suture zone was formed due to the northward and southward subduction of the Solonker ocean lithosphere beneath the Paleozoic South Gobi and Manchuride arcs respectively. and overlying the
Silurian~Permian
as Hashaat/Atasbogd and
arc is defined
Carboniferous granites abundant in the arc massif
Enshoo/Nuhetdavaa
fragments (even small ones!) never cut generally
eastern
amagmatic the Devonian and Permian flysch.
continuation of the arc is known as the Uliastai
Permian volcanic rocks constituting the arc
active continental margin. We should emphasize
fragments totally disappear across fault boundaries
that the Neoproterozoic basement is intruded by
with Lugingol Formation. In recent terrane-based
granites and overlain by volcanic rocks of the
studies the Lugingol type rocks are interpreted as
same age as those in the neighboring so-called
an overlap assemblage. Strong deformation of the
"arc
Permian and Devonian flysch, the similarity of
terranes.
In
Chinese
back-arc
territory
terranes".
the
Thus,
the
only
difference between "terranes" is a level of erosion
their structural
and the only reason of "terrane' recognition is
inclusions consisting of Upper Carboniferous-
fragmentation the single arc by long and straight
Lower Permian shallow-marine limestones and
faults. Another rock assemblage consists of thick
Lower Carboniferous arc volcanics preclude this
turbidites that is commonly defined as the
interpretation
Ligingol flysch of Permian age. Mongolian
depositional settings if not subduction-accretion
geological maps also show Devonian flysch in the
origin of the flysch.
same zones sometimes along-strike continuations of the
Lugingol
Formation.
Devonian
styles, existence of tectonic
and
indicate
at
least
forearc
In Inner Mongolia, the northern belt of the
and
Lugingol type rocks continue in the Hegenshan 46
International Field Excursion and Workshop on Tectonic Evolution and Crustal Structure of the Paleozoic Chinese Tianshan Urumqi, China, September 9-19, 2009
Mountains where ophiolites tectonically mixed
cleavage/bedding intersections, and wide zones of
with Carboniferous and Lower Permian rocks (Fig.
quartz veins showing en-echelon arrangements.
2). Many researches now agree on subduction-
Kinematic criteria indicate sinistral sense of shear.
accretion nature of this mixture. The southern belt
The same structural style has been observed along
of the Lugingol type rocks of Southern Mongolia
the Mongolian segment of the Solonker suture.
continue to the east as the Solonker suture zone
The melange along it includes blocks of oceanic
(Sengor and Natal'in, 1996). Between these two
and magmatic arc origin, fragments of late
belts there is a narrow strip of Ordovician to
Paleozoic
Permian granitic and volcanic rocks originally
Neoproterozoic
defined as the Sonidzuoqi microcontinent (Fig. 2,
embedded in sheared matrix (Lugingol flysch)
Hsu at al. 1991) and later renamed as the in the
with steep foliation. Observed contacts of the
Baolidao arc. Similar to Southern Mongolia old
inclusions and matrix are
xenocryst ages in Paleozoic granites indicate a
contrary earlier claims for a dominance of nappe
presence of the Precambrian basement.
tectonic
carbonate
styles.
Cambrian
Along the northern boundary of the Sonidzuoqi
platform,
basement.
All
Inclusions
marbles
and
and
of them
universally of the
quartzites
are
steep
Vendianand
~
the
late
unit, sinistral shear zones with steep dipping
Paleozoic carbonates (even large ones
foliation and subhorizontal stretching lineations
have a boudin-like shape with crosscutting
have been observed during the IGCP-480 field
relationships of the internal fabric and boudin
excursion in 2007 (location 3 in Fig. 2). Several
margins. Regardless of the validity of age
zones of shearing confined to the margins of the
determinations these structural features show
unit are shown in the Geological map of Inner
strong shearing. Kinematic criteria indicate that
Mongolia (Liqiao Zhang, 1982) however the
the melange formation have to be explained by
kinematics of this shearing remains unknown.
arc-parallel
Taking into consideration the narrowness of the
subduction or "terrane" accretion.
strike-slip
faulting
20x4 km)
rather
then
Sonidzuoqi unit, the similarity of its arc-related
In the central part of Sothern Mongolia (2 in Fig.
magmatic record with those in Southern Mongolia,
2), a wide shear zone separates the basement of
and following Sengor and Natal'in (1996) we
the South Gobi unit (Tsagaan Uul cratonic
interpret the Sonidzuoqi unit as a strike-slip sliver
"terrane" of Badarch (2005)) and the Lugingol-
of the Uliastai active continental margin. This
Hegenshan unit. The structure of this junction has
sliver entraps a part of the Uliastai forearc region,
a similar structural style as in the Solonker suture
the Lugingol-Hegenshan unit, which originally
zone. The only difference is that melange along
was to the south of it. If this inference is correct
this junction was created by the dextral shearing
the sinistral displacement of the Sonidzuoqi unit
synchronous with
should be more than 600 km.
amphibolite facies metamorphism. This shearing
upper
greenschist -
low
In the eastern part of Sothern Mongolia (1 in
affects rocks as young as the Permian. Timing and
Fig. 2), the steep dipping early Permian Lugingol
kinematics of this shearing better fit those along
flysch also reveals bedding parallel shearing
the Scytho-Turanian Fault, particularly along its
associated with asymmetric steep plunging folds,
segment - the Central Tien Shan Fault. The
NE-striking
sinistral strike-parallel shearing was also observed
cleavage
with
steep
plunging 47
International Field Excursion and Workshop on Tectonic Evolution and Crustal Structure of the Paleozoic Chinese Tianshan Urumqi, China, September 9-19,2009
but only as overprir.ting structures showing a
Reference
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Lugingol Formation consists of slates and thin
Buslov, M., 2006. Oblique rift geometry of the
bedded ,turbidites showing i that evidently a wide
West Siberian Basin: tectonic setting for the
zone of transitional facies was eliminated due to
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strike-slip faulting.
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General structure of the South Gobi unit both in
Natal'in, B.A. and Sengor, A.M.C., 2005. Late
Mongolia and Inner Mongolia may be defined as a
Palaeozoic to Triassic evolution of the Turan and
mega-melange created because of the strike-slip
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shearing. We infer that the mega-melange was
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formed during oblique subduction leading to arc-
202. Sengor, A.M.e. and Natal'in, B.A., 1996.
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Palaeotectonics of Asia: Fragments of a synthesis.
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popular ideas
Paleozoic-early
Tectonic Evolution of Asia. Rubey Colloquium,
Mesozoic tectonic evolution of the region is
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compared with modern tectonic setting of South-
640.
the
following
oblique in
collision
which the
across
Travin,
Eastern Asia. These ideas employing multiple
A.V.
et
aI.,
2001.
40Ar/39Ar
collisions of the above mentioned terranes do not
datirovanie plasticheskikh deformatsii v Irtyshskoi
account for scales of tectonic zones which are
sdvigivoi
active now. For instance, if widths of arc-forearc
(40Ar/39Ar dating of ductile deformation in the
regions in modern arc-trench systems usually
Irtysh strike-slip fault zone (Eastern Kazakhstan)).
exceed 250 km, the widths of reconstructed
Geokhimiya(12): 1-5.
zone
(Vostochnyi
Kazakhstan)
Paleozoic arcs (including microcontinents) can be
Van der Voo, R., Levashova, N.M., Skrinnik,
only 20 to 50 km. Interestingly, descriptions of
L.I., Kara, T.V. and Bazhenov, M.L., 2006. Late
rocks
and
orogenic, large-scale rotations in the Tien Shan
"microcontinents" do not imply even a medium
and adjacent mobile belts in Kyrgyzstan and
strain.
Kazakhstan. Tectonophysics, 426(3-4): 335-360.
constituting
these
"micro-arcs"
48